Moving-coil electrodynamic motor for a loudspeaker, loudspeaker and pole piece

ABSTRACT

The invention relates to a moving-coil electrodynamic motor and a loudspeaker. A moving-coil includes a winding with a given number of turns. A motor comprises a magnet arranged between a front pole piece and a rear pole piece. The front pole piece and the rear pole piece enclose a magnetic field in a gap and the moving coil is arranged in the gap. The gap may include a groove arranged essentially parallel to the turns. The coil has a height less than or equal to the height of the gap and the groove forms a recessed zone with an internal ring made from electrically conducting material.

PRIORITY CLAIM

This application claims the benefit of French Application No. 02/01782,filed Feb. 13, 2002. The disclosure of the above application isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to a moving-coil electrodynamic motor, and moreparticularly, to a moving-coil electrodynamic motor for a loudspeaker.This invention also relates to a loudspeaker and a pole piece.

2. Related Art

To limit harmonic distortion on reproduction, loudspeakers should havegood response linearity. Linearity is obtained particularly when a coilintercepts a homogeneous magnetic field flux during movement of thecoil. The distance of a maximum linear displacement of a coil may bereferred to as a maximum linear excursion or X_(max), which can beabbreviated to X_(M). Linearity also may be obtained by at least twoother methods. One method involves making the coil a homogeneous windingwith a considerable height (along the front-rear axis of the coilmovement) greater than the height of the gap where the coil is disposed.In this way, as long as a coil remains entirely in the gap, force andcurrent flowing in the coil remain proportional. This configuration,known as a long coil configuration, is suitable for boomers. The secondmethod relates to reducing the height of a coil relative to the heightof a gap. This configuration, known as a short coil configuration, isoften used for tweeters and may be used for mediums.

Despite improvements to reproduce characteristics of loudspeakers havinga moving-coil electrodynamic motor, asymmetry of magnetic field towardboth ends of a gap undermines linearity and eventually affects themaximum linear excursion of a coil. In addition, loudspeakers havesources of sound distortion, such as complex electromagnetic phenomenacreated by variable electric fields, such as displacement of conductorsin the magnetic field, modulation of the static magnetic field of amagnet in a gap by variable current flowing a coil, and a coil “DCshift” and generation of Foucault current. Accordingly, there is a needfor a loudspeaker system that overcomes the foregoing drawbacks.

SUMMARY

The invention relates to a moving-coil electrodynamic motor for aloudspeaker. The moving coil electrodynamic motor includes at least onemagnet having two magnetic poles, a front pole piece and a rear polepiece having the magnet disposed therebetween. A gap is defined on afirst side by a first edge of the front pole piece and on a second sideby a second edge of the real pole piece. The front pole piece and therear pole piece enclose a magnetic field of the magnet in the gap, wherethe gap is configured to be split into two zones. The gap has at leastone edge.

The motor further includes a moving coil and a groove. The moving coilmay be formed by winding an electric conductor to form a specific numberof turns. The electric conductor is connected to an acoustic diaphragm.The moving coil is disposed in the gap and the turns are perpendicularto the magnetic field so that when a current flows through the coil, thecoil moves along a front-rear axis. The groove is formed on the edge ofthe gap and is disposed substantially parallel to the coil. The edge ofthe gap includes a first surface having height E1. The first surface isseparated from a second surface having E2 by the groove having height C.The groove forms a zone, which is receding from the first surface andthe second surface. The first surface is located rearward, and thesecond surface is located forward relative to the groove. Height E1relates to a first gap space of a rear magnetic field B1 and height E2relates to a second gap space of a front magnetic field B2. The coil isconfigured to have height HB less than or equal to height E1+C+E2 of thesplit gap.

Other systems, methods, features and advantages of the invention willbe, or will become, apparent to one with skill in the art uponexamination of the following figures and detailed description. It isintended that all such additional systems, methods, features andadvantages be included within this description, be within the scope ofthe invention, and be protected by the following claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be better understood with reference to the followingdrawings and description. The components in the figures are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention. Moreover, in the figures, likereferenced numerals designate corresponding parts throughout thedifferent views.

FIG. 1 shows a loudspeaker including a motor having a cone-shapediaphragm and a crown magnet.

FIG. 2 shows a loudspeaker including a motor having a dome-shapediaphragm and crown magnet.

FIG. 3 shows a loudspeaker including a motor having a core magnet.

FIG. 4 shows an enlarged view of a loudspeaker.

FIG. 5 shows a detailed view of an optimized loudspeaker of FIG. 4.

FIG. 6 shows a loudspeaker having reduced magnetic loss.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Loudspeakers generally include a motor having a magnet. A magnet has twopoles that are configured to produce a magnetic field. The magneticfield is enclosed in a gap between two pole pieces. Each pole piece isrelated to one of magnet poles. The pole pieces are generally made ofsoft iron or low carbon content steel. The gap is a free space zonewhere the magnetic field is substantially constant. The gap is alsowhere the pole pieces are closest to each other. A moving coil is formedby turns of a conductive wire and disposed in the gap. When currentflows through the coil, the coil is subject to magnetic field and forceis generated which results in movement of the coil in accordance withvector formula F=B·I·l (where B is the intensity of induction ormagnetic field, I is the intensity of current and l is the length ofconductors subject to magnetic field). A minimum width of the gapdepends on the thickness of the coil because the coil needs to movewithout interference in the gap. Accordingly, sufficient clearance ofthe gap is needed to allow free movement of the coil. Other factors suchas constraints of manufacture, expansion of materials, etc. should betaken into account to determine the minimum width of the gap.

In order to convert force moving the coil into an acoustic pressurewave, the coil may be connected to an acoustic diaphragm that can movethe air in its enclosure. The structural and dimensional characteristicsof a diaphragm depend upon loudspeakers using such diaphragm. A motorhas a circular symmetry with respect to a central axis extending fromfront to rear and the coil also is made of a circular winding of turns.Thus, the diaphragm may be circular, but an elliptical diaphragm is alsopossible.

For reproduction, the frequency range of audible sound for human ear andeven somesthetic sound having the lowest frequency is generally verywide, for example, approximately between 20 Hz and 20 kHz. Because thesound wavelengths vary greatly on this wide frequency range, a singleloudspeaker may not be able to reproduce sound having good reproductioncharacteristics, such as less distortion, high sensitivity anddirectivity across the entire frequency range. Accordingly, loudspeakershave been configured with reduced frequency range to obtain goodreproduction characteristics over that reduced range. Specifically,loudspeakers are configured to be adapted to low frequencies, highfrequencies, and intermediate frequencies, and these loudspeakers areknown as boomers or woofers, tweeters, and mediums, respectively.

A diaphragm used for boomers has an extended surface and responds to alarge movement or excursion, whereas a diaphragm of tweeters has areduced surface. Difference in diaphragm sizes between boomers andtweeters led to different technical constructions of a motor. A generaldescription of different types of loudspeakers is found in “Techniquesof loudspeakers and speakers (Techniques des haut-parleurs & enceintesacoustiques),” written by Pierre Loyez and published by DUNOD.

For a boomer, a diaphragm has a cone shape and a motor magnet has a ring(crown) shape and is generally a ferrite type. A gap is formed betweenan inner edge of a front pole piece disposed on a front pole of themagnet and a central core piece of a rear pole piece. A magnetic fieldis enclosed in the gap. The central core piece extends in a forwarddirection from a rear pole piece that is related to a rear pole of themagnet. One end of the diaphragm is connected to a coil and the otherend of the diaphragm is connected to one edge of a first suspension. Thefirst suspension is disposed in a peripheral area of the motor andsecured to a rigid frame that is fixed to the motor. A second suspensionor spider is disposed in the gap and allows the coil to be securedaxially during its front-rear movements. Accordingly, the coil remainsfree, without lateral friction, in the gap. This double guidancestructure by the first suspension and the spider allows the coil toavoid interference even in the case of large excursions of the coil. Acore cover or dust cover can be used at the center of the cone.

For a tweeter, a diaphragm may have a dome shape, which may be concaveor convex. A core magnet may have a shape of a metal cylinder and ismade of neodymium, iron, or boron. The core magnet is disposed on a rearpole piece or a yoke, which has a first magnetic pole. The magnet may bealso a ferrite type or a metal alloy type such as Ticonal. The rear polepiece has a lateral protrusion from rear to front and encloses amagnetic field in a gap by the lateral protrusion. A front pole piece isa plate or a pellet and is disposed adjacent a second magnetic pole. Anedge of the front pole piece defines the gap along with the yoke. Aperipheral suspension of the dome holds a moving coil along a front-rearaxis during movements of the coil inside the gap without the lateralfriction. This structure is known as a single guidance structure.Accordingly, a tweeter has a single guidance structure, as opposed to adouble guidance structure for a boomer as previously described.

A medium combines the characteristics of a boomer and a tweeterdescribed above. Specifically, a medium may have a dome shaped diaphragmand a ferrite ring magnet. The magnetic field may be enclosed in a gapby a protrusion of a rear pole piece toward front.

As previously stated, it is possible to classify loudspeakers accordingto the structure and material of a magnet or shape of diaphragm usedtherein. The structure of a magnet may be either a solid core magnet ora magnet having a central opening. For a magnet having the centralopening, a crown or ring magnet is disposed outwardly relative to acentral axis and a coil. For a core magnet, a coil is disposed outwardlyrelative to a magnet. For a crown magnet, a coil may be disposedinwardly relative to the magnet toward the center. A magnet may be aferrite type magnet; a metal alloy type magnet such as Ticonal, Alnicoor ALCOMAX® (aluminum, nickel, titanium, cobalt, iron); or alternativelyor additionally, a rare earth magnet (samarium, cobalt, neodymium, iron,boron). The shape of the diaphragm may be dome, cone, or mixed shapesuch as W shape.

FIG. 1 shows a loudspeaker 1 including an electrodynamic motor 100. Themotor 100 has a crown (ring) magnet 3, and a cone-shaped diaphragm 6.The loudspeaker 1 is adapted to reproduce low frequencies of sound,i.e., a boomer. The crown magnet 3 may be a ferrite type having twoopposed magnetic pole surfaces. The crown magnet 3 rests on a rear polepiece (a real plate) 4 facing a rear pole surface. The rear pole piece 4has a protrusion 4′ extending through a central opening of the ringmagnet 3. A front pole piece 2 is disposed on a front pole surface ofmagnet 3. This front pole piece 2 has a crown or ring shape with acentral opening like magnet 3. A gap 110 is formed between the inneredges 103 of the front pole piece 2 and a corresponding zone 104 of theprotrusion 4′ of the rear pole piece 4. The two pole pieces 2, 4 are,for example made of soft iron.

The inner edges 103 of the pole piece 2 include a grove 105. In the gapzone, another groove 106 may be formed on a front protrusion 4′ of therear pole piece 4. Splitting the gap 110 relative to the groove 105results in two zones having substantially equal height and defining afirst field zone and a second field zone in the gap, such as zones 36,37 described in conjunction with FIG. 4. The first field zone is moreproximal to a rear side of the motor 100 than the second field zone. Theshape of the split gap may be adapted to a specific structure thatoptimizes operations of a loudspeaker. In particular, heights of thefirst and second field zones may be different. Also, the grooves 105,106 may be or may not be symmetrical. Alternatively, only one of grooves105, 106 may be formed. In FIG. 1, the edges 103 of the pole piece 2,which are a starting point of enclosing magnetic field in the gap 110,are shown to have a reduced thickness compared with other dimensions.The thickness of the edges 103 of the pole piece 2 corresponds to theheight of the gap 110. Different values for the height of the gap 110may be available, as will be described below. The height of gap 110changes depending on the material used and the magnetic field generatedby magnet 3 to avoid a magnetic saturation of such material.

A coil 5 includes turns of an electric conductor and is disposed in thegap 110. The coil 5 is homogeneous because there is the same number ofturns at each level along a height of the coil 5. As a result, the forcegenerated by a given magnetic field is constant at every point of theheight of the coil 5. As shown in FIG. 1, the height of the coil 5 isless than the height of the gap 110. At rest, in the absence of current,the coil 5 is located at the same level with the groove 105 and a frontend of the coil 5 intercepts a front magnetic field and a rear end ofthe coil 5 intercepts a rear magnetic field. At rest, disposition of thecoil 5 may be or may not be symmetrical with respect to the groove 105along a front-rear axis (Z-Z) 120. When current is present, the coil 5moves along the front-rear axis (Z-Z) 120. If the coil 5 is disposedsymmetric to the groove 105, the coil height intercepting the rearmagnetic field is equal to the coil height intercepting the frontmagnetic field. By contrast, if the coil 5 is not symmetrically disposedto the groove, the coil heights intercepting the rear magnetic field andthe front magnetic field may be different from each other.

A first end 101 of the cone-shaped diaphragm 6 is attached to a coilsupport 5′ that forms part of the coil 5 A second end 102 of the coneshaped diaphragm 6 is attached to a peripheral suspension 8. Theperipheral suspension 8 is secured to a rigid frame 7, which is securedon the motor 100. An inner suspension or “spider” 10 holds the first end101 of the cone-shaped diaphragm 6 that is coupled to the coil 5, sothat the coil 5 is not rubbed against other elements of the motor 100during front-rear movements. A dust cover 9 is disposed towards a centerof the cone-shaped diaphragm 6.

FIG. 2 shows a loudspeaker 11 including a ring (crown) magnet 13, afront pole piece 12, and a rear pole piece having two parts 14, 14′.However, since the frequency range to be reproduced is in the middlepart of sound spectrum, a diaphragm 16 has a dome-shape that isperipherally connected to the coil 15 disposed in a gap 210. The magnet13 may be a ferrite type magnet. The dome-shaped diaphragm 16 and thecoil 15 are connected to a chassis 17 by a peripheral suspension 18.Absorbent materials 19 are disposed to reduce damping of the system. Aprotrusion 14′ of the rear pole piece 14 provides an added ring, whichis open at its center for the passage of connecting leads 40 to the coil15. The connecting leads 40 pass through the chassis 17 of a frame. Therear pole piece 14 and the front pole piece 12 may be ring shaped, aredisposed at each pole of the magnet 10 and may be made of metal, such assoft iron. Inner edges 202 of the front pole piece 12 define the gap 210and include a groove 205. The groove 205 splits the gap 210 into twozones. An edge 204 of the protrusion 14′ of the rear pole piece 14 alsodefines the gap 210 and has a groove 206. The groove 205 can split thegap 210 into two zones having substantially equal height in order todefine a first field zone and a second field zone of the gap as zones36, 37 in FIG. 4 below. The first field zone is more proximal to a rearside of the motor 200 than the second field zone. Alternatively, the twozones may not have the equal height. The shape of the split gap may beadapted to specific structure that can optimize an operation of theloudspeaker. In particular, heights of the first and second field zonesmay be different. Similarly, where grooves 205, 206 are present on eachof the two edges 202, 204 defining the gap 210, the grooves 205, 206 mayor may not be symmetrical. Alternatively, only one of the grooves 205,206 may be formed.

FIG. 3 shows another loudspeaker 21 having a moving-coil electrodynamicmotor 300. Referring to FIG. 3, a magnet 23 is a core magnet, which isdisposed at the center of the motor 300. The magnet 23 may be a pelletor a cylindrical ring and may be a rare earth type magnet. Alternativelyand additionally, the magnet 23 may be made of ferrite; aluminum,nickel, titanium, cobalt and iron based alloy; Ticonal, Alnico orALCOMAX®. The magnet 23 includes two poles and rests on a rear polepiece or yoke 24. The rear pole piece 24 encloses a magnetic fieldacross a gap 310 on edges 302 of a front pole piece 22. A moving coil 25is disposed in the gap 310. The gap 310 is split in two zones by agroove 305 formed on the edge 302 of the front pole piece 22.Symmetrically, a groove 306 is also made on the rear pole piece 24.Alternatively, the grooves 305, 306 may be omitted, or the gap 310 maybe split in two zones only on one side. Further alternatively, the motor300 may be a central opening.

In FIGS. 1, 2 and 3, the motor, the magnet, the two pole pieces, and thecoil have a circular symmetry relative to a central revolution axis 320that extends from a front direction to a rear direction. However,non-circular structure is also available.

FIG. 4 shows structure of a motor 400 in detail. Only the right-handpart of the motor 400 is shown along a front-rear axis of symmetry 31. Amagnet 33 is a core magnet which has a central opening along the axis31. Through this central opening, elements of the motor 400 may beconnected, for example, by injecting a plastic material in a hot stateor a resin. The central opening allows contained air to pass through anddecompress. Alternatively, the magnet 33 may be solid without thecentral opening as shown in FIG. 3. The magnet 33 has a first front polesurface where a front pole piece (or field plate) 32 is placed. Thefront pole piece 32 has a center opening but alternatively, as in FIG.3, it may be solid without the opening.

The magnet 33 has a second pole surface 46 opposite to the first polesurface—45 and where a rear pole piece or yoke 34 is disposed as shownin FIG. 4. The rear pole piece or the yoke 34 has a forward protrusion34′ designed to enclose magnetic field in a gap 410. The rear pole piece34 is open toward its center, but alternatively, as in FIG. 3, it may besolid. The gap 410 is defined by an edge of the front pole piece 32 andthe corresponding surface zone of the protrusion 34′ of the rear polepiece 34, which is opposite to the edge of the front pole piece 32. Agroove 30 may be formed on the edge of the front pole piece 32 to splitthe gap 410 into two field zones. The groove 30 has a height C.

The groove 30 is formed on the edge of the front pole piece 32 so thatheight E1 of a rear field zone 37 is substantially equal to height E2 ofa front field zone 36. Alternatively, these heights by optimizationcomputing methods may be different depending on the materials andstructure used. The groove 30 is internally surrounded by anelectrically conductive material, for example, copper or graphitecarbon. A base wall 30′ of the groove 30 is round and connectionsbetween the base wall 30′ and side walls 30″ (top and bottom walls here)of the groove 30 are round. In fact, a ridge or a cornered connection onsurfaces of a magnetized pole piece creates singular points indistribution of magnetic field, which may cause an adverse effect.

To reduce costs, the front pole piece 32 having the groove 30 may beformed from at least two elements. A two piece groove 30 makes it easierto place a closed ring of conductive material in the groove 30. Forexample, the front pole piece 32 with the groove 30 is made of twoelements, a first element corresponds to the front field zone 30 of(height E2) and a second element corresponds to the groove 30 and therear field zone 37 of (height E1). The conductive ring is then insertedin the second element at the groove level, and then the first element isplaced on this assembly. Various arrangements to facilitate dispositionof the ring both in the front pole piece and the rear pole piece may bepossible. In particular, the two elements may, for example, be identicalor substantially identical to height E1+C/2 and E2+C/2 respectively.

A coil 35 is disposed in the gap 410 and its height is preferably lessthan height of the gap 410. Alternatively, the height of the coil 35 maybe equal to the height of the gap. A coil support forming a part of thecoil 35 (not shown) is normally attached forwardly to a diaphragm havinga cone or dome shape depending on types of loudspeakers. At rest, thecoil 35 is disposed at the same level with the groove 30. The motor 400is configured that the height of the front part of the coil 35intercepting the front field zone 36 is substantially equal to theheight of the rear part of the coil 35 that intercepts the rear fieldzone 37. Thus, E1=E2=E, and a height H_(B) of turns of the coil 35 ispreferably equal to E+C, which permeits a maximum excursion X_(M) of thecoil 35 with a good linearity to +/− ((E/2)+C) relatively to the restposition of the coil 35. At rest, the rear end of the turns of the coil35 is at the same level of the mid-height of E1 and the front end of theturns of the coil 35 is at the same level of the mid-height E2.

As previously stated, optimization tools such as MAGNET® or OPERA® haveenabled the motors to be adapted to specific range of frequency. Forexample, heights E1 and E2 may be different and therefore, the groove is30 disposed offset with respect to the mid-height of the gap. The widthof the gap may be different in the rear field zone 36, the groove 30, orthe rear field zone.

FIG. 4 further shows an additional structure in broken lines. The frontpole piece 32 may protrude forwardly (39). The protrusion 34′ of therear pole piece 34 may not extend in the same way as the protrusion 39,and the front field zone 36 may be only slightly modified. However, theprotrusion 39 has a frusta-conical edge 40 to reduce modification of thefront field zone 36 resulting from the protrusion 39. Similarly, theprotrusion 34′ of the rear pole piece 34, may have a front free end 38and the protrusion 34′ is generally frusta-conical. Alternatively oradditionally, the modification to the protrusions 34′ and 39 may be madein combination. Due to the modification of the protrusion 34′, a forwardend of the protrusion 34′ may not exceed a front end of the front fieldzone 36. The free end 38 may not reach the front end of the front polepiece 32. Alternatively or additionally, the free end 38 may end at thesame level as E2. This split gap structure with a front pole piecehaving an edge and a groove is adapted to various types of loudspeaker(dome or cone) as was described in connection with FIGS. 1 to 3.

FIG. 5 shows optimization of the motor 400 in FIG. 4. In FIG. 5, theprotrusion 39 of the front pole piece 32 may be changed to havefrusta-conical shape 40 and thus, the width of the gap may not be thesame. Specifically, the width of the gap may be greater toward rear thanfront so that the magnet 33 is closer to the rear field zone 37 than tothe front field zone 36. Alternatively or additionally, one edge or bothedges defining the gap 410 may be inclined to produce the same effect,unlike the structure shown in FIG. 5 where edges defining the gap 410are parallel to each other. A coil that is not homogeneous may be usedto create differences in magnetic fields depending on the gap zone. Theproduct B.l can be constant at every point of the coil 35 both in staticstate and during its movements. Reduced forces (the number of turnsdecreasing at one end of the coil), may be compensated by addingequivalent forces (the number of turns increasing at the other end ofthe coil). Thus, any type of coil, i.e., homogenous or not, may be usedby adjusting magnetic field according to the height of the gap.

FIG. 6 shows a loudspeaker 600, which achieves low magnetic loss. Aloudspeaker 600 may be suitable, in particular, for appliances in whichmagnetic field likely disturbs operations, such as CRT of a televisionset, a magnetic resonance measurement device (RMN), or for articleshaving the risk of “demagnetizing” such as a computer disk or magnetictapes of a cassette for a cassette player or a magnetic track payment ortransportation card. A motor 610 shown in FIG. 6 has a core magnet 53.Additionally, the motor 610 may have a counter-magnet 60 on a frontprotrusion 59 of a front pole piece 52. Using a counter-magnet 60improves efficiency because it enables magnetic field lines to be betterchanneled in a gap 620 where a coil 55 is located. At rest, the coil 55intercepts by its rear part a rear magnetic field and by its front part,a front magnetic field. The rear magnetic field and the front magneticfield are disposed in the gap 620 split by a groove 50. Main magneticfield of the motor 610 is generated by the magnet 53. P+ and P− indicatetwo magnetic poles having opposite polarity (north and south orvice-versa) of each magnet 53 to show that magnet 53 and counter-magnet60 are oriented in opposition. The front and rear pole pieces 52, 54,the magnet 53 and the counter-magnet 60 are symmetric with respect to acentral axis 51. The motor 610 circularly revolves. Alternatively, acentral opening may be located at the center of the motor. A rear polepiece 54 or a yoke encloses magnetic field generated by the magnet 53 inthe gap.

As shown in FIG. 6, the magnet may be a core magnet. This core magnetassembly includes a forward protrusion 54′, 54″ and 54′″. A pellet 61made of ferromagnetic material disposed in front of the counter-magnet60 also enables magnetic field to be enclosed by an end of protrusion54′″ of the rear piece pole 54. As a result, loss of magnetic field maybe substantially reduced. The space between an edge of the pellet 61 anda corresponding edge of 54′″ is smaller than the minimum width of thegap because only the coil support (not shown) is disposed at this level.However, if coil excursion is very large until it reaches the level ofthe pellet 61, sufficient space should be left for the coil 55 to beable to move in the gap without any interference. In one example, thefront pellet 61 and the protrusion 54′″ of the rear pole piece 54 may beomitted. In another example, the front protrusions 59 of the front polepiece 52 may be omitted and the counter-magnet 60 is disposed directlyon the front pole piece 52. Alternatively or additionally, the parts54′″ and 54″ of the rear pole piece 54 may be omitted. The foregoingexamples may be combined with one another.

The counter-magnet 60 makes magnetic field in the gap 620 symmetric orequal between a rear field zone having height E1 and the front fieldzone having height E2. As a result, the difference in the width of thegap between the rear field zone and the front field zone can be reducedand the motor optimized. In fact, the rear field zone, which is closerto the magnet 53, is subjected to greater magnetization than the frontfield zone. In the absence of the counter-magnet 60, the front fieldzone has a greater staggering magnetic field lines. For the same reason,during optimization, the gap in the rear field zone may need to beenlarged more compared with that in the front field zone. By contrast,the counter-magnet 60 allows magnetic field lines to be better channeledin the gap 620 and the magnetization is better distributed between thetwo field zones. Thus, depending on the degree of optimization selectedor presence of the counter-magnet 60, width between the front and rearzones of the gap may differ. The height of the gap is measured relativeto the central axis 51, which is substantially parallel to the movementof the coil 55, and the width of the gap is the difference separatingthe front pole piece edge and the corresponding rear pole piece edge.Although FIG. 6 describes the motor with the core magnet, a crown magnetalso may be used. Likewise, a groove may be formed on each of the twoedges of the gap or may be disposed on the other pole piece, i.e., therear pole piece 54 instead of the front pole piece 55.

As described above, an electrodynamic motor coil includes a moving coilformed by a predetermined number of turns of an electric conductor. Theelectric conductor is connected to an acoustic diaphragm. A motorincludes at least one magnet having two magnetic poles and disposedbetween a front pole piece and a rear pole piece. Two poles haveopposite polarities. The front pole piece and the rear pole pieceenclose a magnetic field formed from the magnet in a gap. The gap may bedefined on a first side by a first edge of the front pole piece and on asecond side by a second edge of the rear pole piece. A moving coildisposed in the gap turns perpendicularly to the magnetic field so thatwhen current flows the coil, the coil moves along a front-rear axis.

The gap may be split into two zones and at least one of edges of the gapmay have a groove. The groove is disposed substantially parallel to theturns and the edge has a first rear surface having height E1 separatedfrom a second front surface having height E2 by the groove having heightC. The groove forms a zone receding from the first surface and thesecond surface. The first rear surface defines a first gap space of rearmagnetic field B1, and the second rear surface defines a second gapspace of front magnetic field B2. The coil has a height H_(B) less thanor equal to height E1+C+E2 of the split gap.

The groove internally has a continuous and closed ring of anelectrically conductive material along its walls. The ring of conductivematerial may occupy all or part of the groove. Width of the gap may beor may not be identical along the height of the gap. The first surfacemay recede from the second surface. Similarly, the groove may bedisposed on an edge of the front pole piece and an edge of the rear polepiece. The groove may be or may not be symmetrical with respect to thegap. Winding of the coil may be or may not be homogeneous. Height E1 maybe or may not be equal to height E2.

At rest, the coil is disposed facing the groove and intercepts by afirst end with NB1 turns, the rear magnetic field B1 and by a second endwith NB2 turns, the front magnetic field B2. Thus, the product B1·NB1 issubstantially equal to the product B2·NB2 along the height of the coil,where E1 is substantially equal to E2, the winding of the coil issubstantially homogeneous and the coil has a height H_(B) that issubstantially equal to E1+C or E2+C or E1/2+C+E2/2 (an average of twoprevious formulae).

The gap has a width that is substantially constant at least alongheights E1 and E2. The maximum linear excursion X_(M) of the coil oneither side of the rest position is substantially equal in absolutevalue to (E1/2)+C or (E2/2)+C or an average of the two. The coil issubstantially homogeneous and E1=E2=E and H_(B)=E+C. The maximumexcursion X_(M) of the coil on either side of the rest position is inabsolute value X_(M)=(E/2)+C. The groove may be disposed on the firstedge of the front pole piece and on the second edge of the rear polepiece. The front pole piece protrudes forwardly, and the rear pole piecedoes not protrude forwardly in response to the protrusion of the frontpole piece.

Free end of the rear pole piece adjacent the gap zone may be conic.Further, the front protrusion of the front pole piece may be conictoward its peripheral edge. The motor may include at least onecounter-magnet toward front. The counter-magnet has the poles orientedin the opposite way to the orientation of the poles of the magnet. Themotor also may include a pellet made of a ferromagnetic material on thecounter-magnet and the rear pole piece may protrude forwardly. Theforward protrusion of the rear pole piece may be as far at maximum asthe top level of the pellet. The ferromagnetic material is soft iron.

The motor is symmetric with respect to a central axis extending fromfront to rear and circularly revolves. Edges of the front and the rearpole pieces may be straight and parallel to one another, andalternatively or additionally, they may be straight and inclined. Theside of the gap without the groove is straight and substantiallyparallel to the front-rear axis. The first field zone (height E1) issubstantially equal to the second field zone (height E2) so that thewidth of the gap is substantially constant along its height. The widthof the gap at the point having the first rear magnetic field B1 islarger than the width of the gap at the point having the second frontmagnetic field B2.

The magnet may be a core magnet, and the front pole piece may be asubstantially flat pellet. The rear pole piece may be a yoke having a Ushape. The yoke may have a base on which the magnet rests. The split gapis defined by the edge of the pellet and the upright edge of the yoke.E1 and E2 each may be approximately 3 mm, C may be approximately 4.175mm, the protrusion of the front pole piece may be approximately equal to3 mm and H_(B) is approximately 7.025 mm. Free end of the rear polepiece adjacent gap zone may be lowered with respect to a front end ofthe front pole piece having height E2. For example, the free end of therear pole piece may be lowed by about 0.5 mm. The width of the gap inthe rear zone defined by E1 is larger than the width of the gap in thefront zone defined by E2. The width of the gap in the zone of the groovewith the electrically conductive material is intermediate value comparedto the width of the gap at E1 and E2.

The following is an example of a motor incorporating concepts disclosedabove. One of skill in the art will recognize that many other examplesare possible based on the teachings here. A diameter of the magnet maybe 37 mm and a thickness may be 6 mm. The edge E1 of the gap is on adiameter of 37 mm and the edge E2 of the gap is on a diameter of 37.5mm. The edge C of the material is on a diameter of 37.40 mm. The inneredge of the groove is on a diameter of 22.3 mm. The edge of the gap onthe rear pole piece is on a diameter of 40.7 mm. The inner edge of thecoil is on a diameter of 38.0 mm and the outer edge of the coil is on adiameter of 40.2 mm. The clearance for the coil with respect to each ofthe two edges of the gap is about 250 μm. The conic free end of the rearpole piece adjacent the gap zone is approximately 27.5° with respect tothe horizontal. The outer edge of the rear pole piece is on anapproximate diameter of 50 mm. The dimensions described above are by wayof example only and various dimensions are possible due to constraintsof manufacture, machining, molding, etc.

The magnet may be a crown magnet. The front pole piece may be asubstantially flat ring having an inner peripheral edge toward thecenter of the crown. The rear pole piece is formed by a rear plate onwhich the magnet rests and a central core extending from rear to front.The split gap is defined by the inner edge of the ring and the centralcore zone. Electrically conductive material is selected from gold,silver, copper, aluminum, graphite carbon or their combination.Preferably, the conductive material is made of copper.

Conductive material is electrically insulated from a grooved pole piece.Alternatively or additionally, the conductive material may not beelectrically insulated from the material of its grooved pole piece. Theelectrical insulator of the conductive material has a high thermalconductivity coefficient. The base of the groove has two connectingzones that are round at the top and bottom walls of the groove. The coilincludes a coil support. The coil may be homogeneous over its height.Alternatively, the coil may not be homogeneous over its height, and twozones of turns may be separated by a space. The turns of the coil areconductive tracks deposited on the coil support.

A magnet may be a core magnet and have a central opening. A front polepiece of the central opening core magnet also has the central opening.The counter-magnet of a core magnet with a central opening may haveanother central opening. The pellet of a counter-magnet with the centralopening also has a central opening. Alternatively, the pellet may be asubstantially continuous and made of a solid flat part. The front polepiece and the rear pole piece are made of a one piece material.Alternatively or additionally, it is also possible that the front polepiece and the rear pole piece may be made from combining at least twoelements.

The motor described above may be suitable for a moving-coilelectrodynamic loudspeaker. A pole piece for a motor described above maybe modified. In particular, the gap is split into two zones and at leastone of the peripheral edges have a first surface having height E1separated from a second surface having height E2 by a groove havingheight C. The groove forms a zone receding from the first surface andthe second surface. The groove may be on the front pole piece and/or onthe rear pole piece.

A moving-coil electrodynamic motor described herein has improvedlinearity. As a result of the split structure of the gap, it is possibleto balance forces to which a coil is subject during its movements andcompensate any discrepancy in forces between two zones defined by E1 andE2. For example, when force created by a field B1 decreases because thenumber of turns exposed decreases, the coil moves forwardly. Anequivalent force created by a field B2 is added because more turns enterthe field B2. Accordingly, the product B.l is adjusted so that there iseffective compensation on movement of the coil. Specifically, the fieldsB1 and B2 may be identical or different depending on whether a coil ishomogeneous or not. This configuration particularly reduces distortionsof odd order, which may be considered the most unpleasant to ears. Thisconfiguration also reduces DC shift effects. Furthermore, the motors maybe extremely compact, while having a large excursion of the moving coil.

The use of a counter-magnet increases efficiency of a motor, and aloudspeaker with low magnetic losses may be particularly adapted totelevision applications where magnetic field should be reduced to avoiddistortions of images produced by cathode ray tubes. Also, by the use ofcomputing tools for modeling magnetic and electromagnetic fields, it ispossible to optimize a basic configuration to obtain even better resultswhile reducing materials to be used. Volumes may be determined so as tobe at the bottom limit of magnetic saturation. With optimization, thewidth of the gap may be different between the rear field zone defined byE1 and the front field zone defined by E2, because E1 is closer to themagnet than E2. Similarly, a front protrusion of the rear pole piece maybe shortened with respect to the front end of E2 which is oppositethereto. The loudspeaker can substantially reduce a distortion, forexample, approximately three times, in a low-frequency range comparedwith conventional loudspeakers.

Any combination of different types of a magnet is possible, regardlessof magnet shape (crown, core, etc.), magnet material (ferrite, metal,rare earths, etc.) and diaphragm type (dome, cone, etc.). The inventionmay use structure for reducing magnetic radiation, such as shielding andone or more counter-magnets. Sensors for providing information to anamplifier to which the sensors are connected may be used to control theamplifier.

While various embodiments of the invention have been described, it willbe apparent to those of ordinary skill in the art that many moreembodiments and implementations are possible within the scope of theinvention. Accordingly, the invention is not to be restricted except inlight of the attached claims and their equivalents.

1. A moving-coil electrodynamic motor for a loudspeaker comprising: amagnet having two magnetic poles; a first pole piece and a second polepiece defining a gap and enclosing a magnetic field in the gap where themagnet is disposed between the first pole piece and the second polepiece and where the gap includes a first zone and a second zone; amoving coil including a winding of a specific number of turns of anelectric conductor and disposed in the gap, where the turns areperpendicular to the magnetic field so that the coil moves in responseto a current along a front-rear axis; and, a groove disposedsubstantially parallel to the turns and including an electricallyconductive material, where the groove has a height C and separates thefirst zone having a height E1 and the second zone having a height E2;and, where when E1 is substantially equal to E2, the winding of the coilis substantially homogeneous and the moving coil has a height H_(B) thatis substantially equal to E1+C, E2+C or E1/2+C+E2/2.
 2. A motoraccording to claim 1, where E1 defines a first gap space of a rearmagnetic field B1 and E2 defines a second gap space of a front magneticfield B2.
 3. A motor according to claim 2, where the coil includes afirst end having NB1 turns and a second end having NB2 turns, and atrest, the first end of the coil intercepts the rear magnetic field B1and the second end of the coil intercepts the front magnetic field B2.4. A motor according to claim 3, where the product B1·NB1 issubstantially equal to the product B2·NB2 along the height of the coil.5. A motor according to claim 1, where the gap includes at least oneedge and the groove is formed on the edge of the gap.
 6. A motoraccording to claim 1, where the motor further comprises at least onecounter-magnet disposed adjacent the first pole piece, thecounter-magnet having two poles oppositely magnetized to the poles ofthe magnet.
 7. A motor according to claim 6, where the motor furthercomprises a pellet made of a ferromagnetic material and disposedadjacent the counter magnet, and where the second pole piece protrudesforwardly to an extent that a protrusion does not exceed the pellet. 8.A motor according to claim 1, where the magnet is a crown magnet and thesecond pole piece is a substantially flat ring having an inner edgetoward a center, and; where the first pole piece includes: a base plateon which the magnet rests; and a central core extending perpendicularlyto the base plate, and where the front zone and the second zone aredisposed between the inner edge of the ring and the central core.
 9. Amotor according to claim 1, where the electrically conductive materialis selected from one of gold, silver, copper, aluminum, graphite carbon,and a combination thereof.
 10. A motor according to claim 1, where theelectrically conductive material is made of copper.
 11. A loudspeakeraccording to claim 1, where the coil includes a first end having NB1turns and a second end having NB2 turns, and at rest, the first endintercepts the rear magnetic field B1 and the second end intercepts thefront magnetic field B2; where the magnet is a crown magnet, and thefront pole piece is a substantially flat ring having an inner edgetoward a center, and; where the rear pole piece includes a rear plate onwhich the magnet rests and a central core extending from rear to front,and the inner edge of the ring of the front pole piece and the centralcore of the rear pole piece define the gap.
 12. A loudspeaker accordingto claim 11, where the product B1·NB1 is substantially equal to theproduct B2·NB2 along the height H_(B) of the coil.
 13. A moving-coilelectrodynamic motor for a loudspeaker comprising: a magnet having twomagnetic poles; a first pole piece and a second pole piece defining agap and enclosing a magnetic field in the gap where the magnet isdisposed between the first pole piece and the second pole piece andwhere the gap includes a first zone and a second zone; a moving coilincluding a winding of a specific number of turns of an electricconductor and disposed in the gap, where the turns are perpendicular tothe magnetic field so that the coil moves in response to a current alonga front-rear axis; and, a groove disposed substantially parallel to theturns and including an electrically conductive material, where thegroove has a height C and separates the first zone having a height E1and the second zone having a height E2; where when E1 is substantiallyequal to E2, the winding of the coil is substantially homogeneous andthe moving coil has a height H_(B) that is substantially equal to E1+C,E2+C or E1/2+C+E2/2; where the magnet is a core magnet and the secondpole piece includes a substantially flat pellet; where the first polepiece includes: a yoke having a U shape and at least one upright edgedisposed adjacent the gap; and a base on which the magnet rests; andwhere the first zone and the second zone are disposed between an edge ofthe pellet and the upright edge.
 14. A loudspeaker having a moving-coilelectrodynamic motor comprising: a magnet having two magnetic poles; afront pole piece and a rear pole piece defining a gap and enclosing amagnetic field in the gap where the magnet is disposed between the frontpole piece and the rear pole piece and where the gap includes a firstzone and a second zone; a moving coil including a winding of a specificnumber of turns of an electric conductor and disposed in the gap, wherethe turns are perpendicular to the magnetic field so that the coil movesin response to a current along a front-rear axis; a diaphragm coupled tothe moving coil and responding to the movement of the coil; and, agroove disposed substantially parallel to the turns and including a ringwhere the ring is continuous and closed and made from an electricallyconductive material, and, where the groove has a height C and separatesthe first zone having a height E1 and the second zone having a heightE2; and where the coil is configured to have a height H_(B) either lessthan or equal to the height E1+C+E2 of the gap.
 15. A loudspeakeraccording to claim 14, where the ring internally surrounds the groove.16. A loudspeaker according to claim 15, where the groove includes abase wall and a side wall and connections between, the base wall and theside wall is round.
 17. A loudspeaker according to claim 14, when thefront pole piece includes a first element and a second element and thering is inserted in the second element at the level of the groove.
 18. Aloudspeaker according to claim 17, where the first element is placed onthe ring inserted in the second element.
 19. A loudspeaker having amoving-coil electrodynamic motor, comprising: a front pole piece havinga first edge and a first pole; a rear pole piece having a second edgeand a second pole where the front pole piece and the rear pole piecedefine a gap having a first field zone and a second field zone andenclose a magnetic field in the gap; and, a moving coil formed by awinding of a specific number of turns of an electric conductor anddisposed in the gap; a magnet creating the magnetic field that isperpendicular to the turns so that the coil moves in response to acurrent along a front-rear axis; and, a groove configured to separatethe first field zone having a height E1 from the second field zonehaving a height E2, the groove having a height C and forming on thefront pole piece where the groove includes a zone receding from thesurface of the front pole piece; where a height H_(B) of the coil isequal to E+C when E=E1=E2, and a maximum excursion of the coil rangesfrom −(E/2+C) to +(E/2+C) relative to the rest position of the coil. 20.A loudspeaker according to claim 19, where the motor further comprisesat least one counter-magnet disposed adjacent the front pole piece, thecounter-magnet having two poles oppositely magnetized to the poles ofthe magnet; where the magnet is a crown magnet, and the front pole pieceis a substantially flat ring having an inner edge toward a center, and;where the rear pole piece includes a rear plate on which the magnetrests and a central core extending from rear to front, and the inneredge of the ring of the front pole piece and the central core of therear pole piece define the gap.
 21. A loudspeaker having a moving-coilelectrodynamic motor, comprising: a front pole piece having a first edgeand a first pole; a rear pole piece having a second edge and a secondpole where the front pole piece and the rear pole piece define a gaphaving a first field zone and a second field zone and enclose a magneticfield in the gap; and, a moving coil formed by a winding of a specificnumber of turns of an electric conductor and disposed in the gap; amagnet creating the magnetic field that is perpendicular to the turns sothat the coil moves in response to a current along a front-rear axis;and, a groove configured to separate the first field zone having aheight E1 from the second field zone having a height E2, the groovehaving a height C and forming on the front pole piece where the grooveincludes a zone receding from the surface of the front pole piece; wherea height H_(B) of the coil is equal to E+C when E=E1=E2, and a maximumexcursion of the coil ranges from −(E/2+C) to +(E/2+C) relative to therest position of the coil; where the moving-coil electrodynamic motorfurther comprises at least one counter-magnet disposed adjacent thefront pole piece, the counter-magnet having two poles oppositelymagnetized to the poles of the magnet; and where the magnet is a coremagnet, and the front pole piece is a substantially flat pellet, and;where the rear pole piece includes a yoke having a U shape and at leastone upright edge and a base on which the magnet rests, and an edge ofthe pellet and the upright edge define the gap.
 22. A loudspeaker havinga moving-coil electrodynamic motor, comprising: a front pole piecehaving a first edge and a first pole; a rear pole piece having a secondedge and a second pole where the front pole piece and the rear polepiece define a gap having a first field zone and a second field zone andenclose a magnetic field in the gap; and, a moving coil formed by awinding of a specific number of turns of an electric conductor anddisposed in the gap; a magnet creating the magnetic field that isperpendicular to the turns so that the coil moves in response to acurrent along a front-rear axis; and, a groove configured to separatethe first field zone having a height E1 from the second field zonehaving a height E2, the groove having a height C and forming on thefront pole piece where the groove includes a zone receding from thesurface of the front pole piece. where a height H_(B) of the coil isequal to E+C when E=E1=E2, and a maximum excursion of the coil rangesfrom −(E/2+C) to +(E/2+C) relative to the rest position of the coil,where the coil includes a first end having NB1 turns and a second endhaving NB2 turns, and at rest, the first end intercepts the rearmagnetic field B1 and the second end intercepts the front magnetic fieldB2; where the magnet is a core magnet and the front pole piece is asubstantially flat pellet, and; where the rear pole piece includes ayoke having a U shape and at least one upright edge and a base on whichthe magnet rests, and an edge of the pellet and the upright edge of theyoke define the gap.
 23. A loudspeaker according to claim 22, where theproduct B1·NB1 is substantially equal to the product B2·NB2 along theheight H_(B) of the coil.
 24. A moving-coil electrodynamic motor for aloudspeaker comprising: a magnet having two magnetic poles; a first polepiece and a second pole piece defining a gap and enclosing a magneticfield in the gap where the magnet is disposed between the first polepiece and the second pole piece and where the gap includes a first zoneand a second zone; a moving coil including a winding of a specificnumber of turns of an electric conductor and disposed in the gap, wherethe turns are perpendicular to the magnetic field so that the coil movesin response to a current along a front-rear axis; and, a groove disposedsubstantially parallel to the turns and including an electricallyconductive material, where the groove has a height C and separates thefirst zone having a height E1 and the second zone having a height E2;and, where when at least one of the first pole piece or the second polepiece is inclined to form a different width of the gap in the first zoneand the second zone, the winding of the coil is not homogeneous and theproduct H_(B)·E1 or H_(B)·E2 is constant.